The positive displacement pump or motor or meter has very wide applications in industry as well as in the domestic area. Many existing versions of the positive displacement fluid handling devices have a small fluid occupied volume compared with the total bulk of the device and consequently, these versions are not suitable to handle fluid movements involving large flow rates. The present invention teaches a positive displacement pump-motor-meter that has a large fluid occupied volume constituting a major portion of the total bulk of the device, which can be constructed into an assembly wherein there is little sliding contact between moving parts and stationary parts included in the device and consequently, the teaching of the present invention provides a highly efficient and powerful positive displacement fluid handling device for pumping fluid or for harnessing power from the moving fluid or for measuring the rate of fluid flow.